1. Field of the Invention
This invention relates to the purification of water, and more particularly concerns compositions for the selective removal of trace levels of toxic metals from water.
2. Description of the Prior Art
Undesirable pollutant species such as dissolved toxic metals can occur regularly or periodically in natural bodies of water such as lakes, rivers, streams, groundwater and stormwater, and in industrial waters such as landfill leachate, municipal sewage systems and wastewater discharges. The toxic metals are primarily multivalent “heavy” metals, and can be caused to form insoluble compounds with anionic additives. Consequently, the most prevalent technique for the removal of the toxic metals is to add to the water a sufficient amount of an anionic precipitating agent. The resultant precipitate is then allowed to settle or is removed by filtration or other methods.
Although precipitative techniques are effective, they require that the water undergoing remediation be accumulated in tanks and held there for considerable periods of time with agitation while the treatment agents are added and dispersed, causing the heavy metals to form precipitates of sufficiently large particle size to facilitate settling and/or filtration. Operations of this nature require large, expensive equipment installation and high operating expense. The high cost of such installations is justifiable only where extremely large volumes of water must be treated on a long term basis, such as in municipal waste water treatment facilities and operations involving large industrial facilities.
Another general technique for removing dissolved metal species from water involves the passage of a stream of the water undergoing remediation through a substantially stationary, water permeable bed of absorbent material. Suitable absorbent materials include ion exchange resins having affinity for species in either cationic or anionic form. The advantage of such technique is that the water can be treated while flowing through a confined bed of the resins, thereby avoiding the need for large holding and processing tanks. Ion exchange resins are generally in the form of spherical beads having been made by catalyzed suspension polymerization of a liquid styrene/divinylbenzene prepolymer mixture. By virtue of controlled conditions of mixing and use of surfactants, the prepolymer is suspended as discrete spherical droplets within an engulfing continuous phase aqueous medium. The resultant product is a polymer in the form of spherical beads having a size generally in the range of 16 to 50 mesh (U.S. Sieve Series), or 1.2 mm to 0.3 mm, respectively. Generally, less than 5% of the product has a size below 50 mesh. The beads are then subjected to chemical treatments which impart specific ion exchange functionality. Because of their bead form and relatively large bead diameter, beds of ion exchange resins in vertically oriented columns or tanks offer relatively little impedance to the flow of water through the bed.
Other specialized absorbents for the removal of trace pollutants from water are available in the form of irregularly shaped granules, the most notable example being granular activated carbon. When comparing absorbents in bead form with absorbents in granule form at the same particle size, beds of granule form absorbents produce higher flow impedance. Even higher flow impedance is encountered when the granules are comprised of water swollen soft polymer, usually referred to as hydrogel polymer and containing between 40% and 70% by weight of water. Examples of the use of hydrogel granules for the selective removal of trace heavy metals from water are disclosed in U.S. Pat. Nos. 7,041,222; 3,715,339 and elsewhere.
The aforesaid hydrogel polymer is generally produced by the thermal condensation polymerization of an aqueous prepolymer solution. The resultant polymer is a large mass which is then subjected to a comminution operation that produces irregularly shaped particles of desired size in the range of 10 to 80 mesh. However, the comminution also produces a significant amount of fines, namely particles having an undesirably small size in the range of 80 to 200 mesh. Said fines are separated from the desired larger granules by a sieving operation. Because of their small size, the fines exhibit faster rates of metals absorption than larger granules of the same composition, but when employed as a bed for the interception of a flow of water to be treated, their high impedance to flow causes a very slow flow rate, or requires a very high pumping pressure. Accordingly, the fines are generally considered to be of little commercial value.
It is accordingly an object of the present invention to diminish the impedance to flow of beds of polymeric absorbent in granular form used for removing dissolved metal species from water flowing through said bed.
It is another object of this invention to provide a composition comprised of absorbent granules, said composition having in bed form diminished resistance to the flow of water therethrough.
It is a further object of the present invention to provide a composition of the aforesaid nature wherein said absorbent granules function to remove dissolved metal species from water.
It is a still further object of this invention to provide a confined bed of the composition of the aforesaid nature.
These objects and other objects and advantages of the invention will be apparent from the following description.